Back in college, I learnt to play the guitar without a formal teacher. I either read lessons online or learnt from my friends. Looking back, I regret not going to a formal teacher because for I internalized bad form and sloppy habits, which I am now having to to painfully untangle from my playing.
The flipside with online lessons or other forms of student directed learning is that you watch an expert do something and try to copy them. The emphasis is on what you ought to do rather than what you should avoid. This gets students to focus more on the outcome rather than the process. We get tunneled into playing a song exactly as we hear it rather than internalizing the right method or technique.
For a student, a directive such as ‘here are the three things you should avoid’ is often more useful than ‘here is how you do it’.